With recent rapid progress of information transmitting systems, silver halide photographic light sensitive materials have been increasingly required to have high sensitivity, Such systems are, for example, high speed phototypesetting systems according to which information output from an electronic computer is immediately displayed as letters or figures by a cathode ray tube and press facsimile systems for rapid transmission of newspaper originals to a remote place.
Todate on the market phototypesetters usually work with a He/Ne laser (632 nm), laser diode (650 nm or 680 nm) or LED (670 nm or 780 nm). Especially phototypesetters that operate with a He/Ne laser or LED are frequently employed, but also argon lasers (488 nm) are still in use.
For a photographic material to be suitable for use in phototypesetting applications it is required that the material is of high speed to so-called high intensity-short time exposure (flash exposure or scanning exposure), namely exposure for 10.sup.-4 second or less and yields images of high contrast and high resolving power.
Photographic phototypesetting materials include photographic films and papers used in a process for preparing a lithographic printing plate and silver salt diffusion transfer based (hereinafter called DTR-) lithographic printing plates disclosed in e.g. U.S. Pat. No. 4,501,811 and U.S. Pat. No. 4,784,933. With the latter materials a lithographic printing plate is immediately obtained without the need of a contact exposure or camera exposure.
A photographic DTR material preferably comprises a silver halide emulsion mainly consisting of silver chloride in order to obtain a sufficiently high rate of solution of the silver halide and a satisfactory gradation necessary for graphic purposes. Only a small amount of silver bromide and/or silver iodide usually not exceeding 5 mole % is present. However as a consequence of the lower energy output of some lasers, e.g. laserdiodes, the sensitivity of these conventional silver chloride DTR emulsions is no longer sufficient when using those types of laser as exposure unit as disclosed in U.S. Pat. No. 5,059,508.
So, in order to obtain DTR materials which can be used for high intensity-short time exposure and which will give good lithographic printing plates it is taught to use an silver halide emulsion with iridium built in as a dopant by adding said iridium during the precipitation and/or the physical ripening of the silver halide and/or a higher silver bromide content as disclosed in e.g. U.S. Pat. Nos. 4,621,041 and 5,059.508.
All these patents teaching the addition of iridium during the precipitation and/or the physical ripening of the silver halide reflect the belief that the activity of the iridium dopant depends enormously on the time of its addition. It has been described in e.g. Journal of the Society of Scientific Photography of Japan, 31, 34(1968) that the addition of small amounts of hexachloroiridate at the precipitation of a silver halide emulsion strongly increases the speed of said emulsion to so-called high intensity-short time exposure due to the iridium ion built in as a dopant in the silver halide crystal. The addition of hexachloroiridate at a later stage in the preparation of a silver halide emulsion has no such effect because the iridium ion is then not longer built in in the silver halide crystal but absorbed to or incorporated into the surface of the silver halide crystal having only a stabilizing effect on the latent image stability.
However, the images or lithographic plates obtained by processing the photographic materials containing an above mentioned emulsion are still not quite satisfactory, especially in respect to minimal and maximal density and contrast.
An image with a low minimal and a high maximal density and a high contrast results in a better sharpness. When the photographic material is to be used in a silver salt diffusion transfer process and especially for preparing printing plates according to this process it is necessary to obtain a silver image with lower minimal and/or higher maximal density and/or higher contrast to improve the printing properties i.a. higher printing endurance, better ink acceptance in the printing areas, less ink acceptance in the non printing areas both during start-up of the printing process (toning) and during printing (staining), sharper prints.
Furthermore, the activity of the built-in iridium complex depends enormously on the conditions during its addition. It is known that the initial ligands of commercially available iridium complexes can exchange with other ligands (water, hydroxyl ions, etc.). This exchange reaction happens whenever the halogen complex is dissolved and the rate is very much depending on the temperature, the acidity, the amount of free silver ions etc. of the medium. So, said activity is therefore unpredictable when the iridium complex is added during the physical ripening and surely during the precipitation of a silver halide emulsion when the above mentioned parameters are constantly changing. So the possibility of adding the iridium complex to the silver halide emulsion under well defined conditions would be a much appreciated improvement.
EP-A 568.091 discloses a process reducing reciprocity failure of a silver chloride emulsion comprising adding irridium and bromide to said chloride emulsion wherein said bromide is added simultaneously or after said irridium. This document does not disclose a method for obtaining an image with a higher contrast. Furthermore, the addition of said bromide is an additional step in the preparation of said emulsion and can easily leads to irreproducible photographic results due to the partial conversion of silver chloride to silver bromide in said silver chloride emulsion.